Apparatus for ionising a gas to obtain high intensity pulsed ions or electrons



Jan. 2, 1962 S. KLEIN APPARATUS FOR IONISING A GAS TO OBTAIN HIGH INTENSITY PULSED IONS OR ELECTRONS Filed Sept. 16, 1959 United States Patent 3,015,745 APPARATUS FOR IONISlNG A GAS TO OBTAIN HIGH INTENSITY PULSED IONS 0R ELEC- TRONS Siegfried Klein, Paris, France, assignor to Commissariat a lEnergie Atomique, Paris, France Filed Sept. 16, 1959, Ser. No. 840,429 Claims priority, application France Sept. 20, 1958 2 Claims. (Cl. 313-63) The present invention relates to processes and apparatus for ionising a gas to obtain high intensity pulsed ions or electrons.

Certain phenomena, more particularly those bound up with the structure of atomic nuclei, are often studied by bombarding certain substances with electrons or ionised particles. Devices knownas ion sources, which are, in common use, are employed to produce these ionised particles. Such sources, however, in addition to being large, have very troublesome disadvantages; in particular," they require considerable energy to supply only very low ion or electron intensities. Regard being had to losses inherent in the very construction of accelerators which subsequently pick up the said particles, the ion intensities which the experimenter obtains on his target are often insufficient.

The present invention contemplates a source of highintensity pulsed ions or electrons which is small and highly eflicient, and supplies a pulsed current of several tens of amperes of ionised particles or electrons.

The said source of pulsed ions or electrons is characterised in that an induction coil and a condenser are suitably connected in parallel and fed with a steady high electrical voltage, and pulses which ionise a gas circulating in an insulating tube situated in the field of the coil are applied to the said gas, which is also made to play the part of an electrical conductor virtually short-circuiting the terminals of the applied high voltage, a heavy current flowing through the said ionised gas.

According to one aspect of the invention, the source comprises a metal gas-inlet tube welded to the mouth of a Pyrex bulb. The other end of the said bulb passes inside a metal chamber in which a vacuum is set up. A steady high electrical voltage is applied to an oscillating circuit consisting of a condenser and an induction coil outside the Pyrex bulb; a spark-gap isolates the coil from the high voltage as long as the condenser is insufiiciently charged. The coil is also connected to the metal gas-inlet tube and to the vacuum chamber; this enables a very heavy current to flow in the coil when the condenser discharses, and the field of the coil ionises the gas circulating in the Pyrex tube; the gas then conducts, and the high voltage applied to the two ends of the said tube causes a considerable increase in ionisation. Furthermore, if consideration is given to the very dense ion plasma to be found at a given instant in the vicinity of the vacuum chamber, it may be likened to a current element similar to a closed circuit; according to Lenzs law, this plasma is subjected to a force of electromagnetic character which facilitates driving it out of the tube, and imparts a high degree of acceleration to it.

A non-limitative example of use of the source of highintensity pulsed ions or electrons to which the invention relates will be described hereinafter with reference to the appended diagrammatic drawing. The arrangements which will be described in connection with this example must be considered as forming part of the invention, it being understood that any equivalent arrangements may inst as well be used without departing from the scope of the invention.

The drawing shows a non-metallic flexible tube 1 a connected to a short metal tube 2. This tube 2 comprises a constriction 3 whereof the function is explained below; it is furthermore welded at 4 to the end of an enclosure consisting of a Pyrex bulb 5, which is open at both ends at gas inlet 4 and gas outlet 6 respectively. The end 6 is tapered to a tubular shape, and passes inside a metal tube 7 which forms part of a chamber 8, also made of metal, in which vacuum may be set up by 'virtue of a pump 9. A flexible annular packing 10 provides a seal between the bulb 5 and the chamber 8. A steady high electrical voltage of the order of 20,000 is applied to the terminals of a circuit consisting of a chargingresistance 11 in series with a condenser 12 having a capacity of the order of a microfarad, the other side of the condenser being connected to the wall of the chamber 8. A spark-gap 13 is connected from the junction of the resistance 11 and the condenser 12 to the tube 2, downstream of the constriction 3. An induction coil 14 with its turns outside the bulb 5 is connected on'the one hand to the spark-gap 13 on the tube 2, and on the other hand to the wall-of the chamber 8. Y

When it is desired to obtain ions, the negative pole of the high voltage will be connected to the chamber 8; on the" other hand, if it is desired to' obtain electrons,

the polarity of the high voltage is simply reversed.

The appliance according to the invention thus operates as follows: a vacuum of the order of 10- mm. of mercury is set up in the chamber 8 with the aid of a pump 9; a small but steady flow of the gas which it is desired to ionise is caused to circulate in the circuit consisting of the tubes 1 and 2, the bulb 5 and the chamber 8. A

steady high voltage of the order of 20,000 is then applied between the resistance 11 and the chamber 8. The resistance 11 enables the condenser 12 to charge up; when the voltage across the terminals of the spark-gap 13 is greater than the breakdown voltage defined by the spacing of its electrodes, the condenser 12 discharges into the coil 14 in a very short time of the order of a microsecond; the electromagnetic field produced by the coil 14 ionises the gas circulating in the bulb 5.

The energy released by the condenser 12 is easily evaluated. Let:

C be the capacity of the condenser 12:0.2 microfarad V be the applied voltage=20,000

then:

E= /2CV /2.2.10 .4.10 =40 joules Under experimental conditions, the discharge lasts for one microsecond. The instantaneous power is therefore:

W=%=%=40 megawatts The considerable power thus brought into play causes the gas contained in the bulb 5 to expand to a large degree and to flow very rapidly into 6; in fact, the constriction 3 in the tube 2 prevents the said gas from returning to the tube 1. A cloud of ionised plasma is therefore to be found in the vicinity of the tube 7; this plasma is the equivalent of an electrical conductor connected to the terminals of the high voltage, and a very high value of ion current is thus obtained; an extractor electrode 15 situated inside the chamber 8 plays the same part as the electrodes used in conventional sources.

The power brought into play when the discharge occurs may be approximately calculated. Let:

V=the applied electrical voltage C=the capacity of the condenser 12 Q=the quantity of electricity brought into play then:

Q coulornbs=C farads x V volts i.e.: Q=0.2X10 2 10 =4.10- coulombs.

Now if the discharge is assumed to last for one microsecond, and having regard to:

Q coulombs =l amperex T seconds.

then:

The foregoing calculations give only an order of magnitude, since in them parasitic capacities, leaks and the resistance of the coil 14 have been neglected. Nevertheless, measurements. have shown the ion intensity obtained in accordance with the invention to be several tens of amperes.

The invention makes. available to experimenters ion sources of such a: kind that, even after passing through the various deflectors and accelerators used, the ion beam still possesses considerable intensity.

I claim:

1. In apparatus for ionising a gas, a conductive-walled enclosure, a source of gas to be ionised, means for maintaining vacuum in the said enclosure, a second insulating-walled enclosure having a gas inlet and a gas outlet connected to said first enclosure and connected to said source of gas to-be ionised, a conductive ele- =4,000 amperes ment at said inlet and a conductive element at said outlet of said second enclosure, an inductive winding around said second enclosure, a condenser, one side of said condenser being connected to one end of said winding, a sparkgap, said spark-gap being connected between the other side of said condenser and a point common to said other conductive element and the other end of said winding, a high-voltage source connected to and charging said con denser and an ion extractor electrode inside said. conductive-walled enclosure.

2. Apparatus. as described in claim. 1, including a constriction in said connection between said insulating-walled enclosure and said source of gas to be ionised whereby the return flow of gas is minimised each time the condenser discharges.

References Cited in the file of this patent UNITED STATES PATENTS 1,004,170 Jacoviello Sept. 26, 1911 1,809,115 Goddard June 9, 1931 2,219,033 Kuhn et al Oct. 22, 1940 2,285,622 Slepian June 9, 1942 2,347,715 Spencer May 2, 1944 2,553,944 Schlesman May 22, 1951 2,764,707 Crawford Sept. 25,; 1956 2,836,750 Weimer May 27, 1958; 2,883,580 Kilpatrick Apr. 21, 1959 2,919,370; Giannini et a1 Dec. 29, 1959 

